1. (a) Field of the Invention
The present invention relates to a method for measuring fundamental frequency component of a fault current or voltage signal, particularly, to a signal processing method for extracting accurate fundamental frequency component from a fault current or voltage signal distorted with a DC-offset, a characteristic frequency component and harmonics, in a relaying apparatus to protect the power system.
2. (b) Description of the Related Art
Distance relays are based on the principles of extracting the fundamental frequency components from voltage and current relaying signals. To extract these components, the prior art has used orthogonal transforms such as Fourier transform. However, in actual fault current or voltage signals, there exist time-varying components, such as a DC-offset, resulting in an error. Thus, these components should be taken into consideration to measure the fundamental frequency component of the fault current or voltage signal accurately.
The prior art has suggested methods for measuring the fundamental frequency components by assuming a specific time constant for the DC-offset. However, this time constant depends on the power system configuration at the moment of a fault and also on the location of the fault. These techniques are prone to produce errors if the time constant of a relaying signal is different from the assumed one. To overcome this drawback, J. C. Gu and S. L. Yu proposes a Fourier filter algorithm (J. C. Gu and S. L. Yu, Removal of DC-offset in current and voltage signals using a novel Fourier filter algorithm, IEEE Trans. Power Delivery, vol. 15, No. 1, January 2000, pp. 73-79), which calculates the DC-offset and then measures the accurate fundamental frequency component by compensating the output of the Fourier filter with the calculated DC-offset. Although this method can measure the fundamental frequency component regardless of the time constant of the DC-offset, it also produces errors when the fault current or voltage signal includes any damped high frequency component.
Whenever a fault occurs on a transmission line, damped resonance frequency components are generated. In the present invention, a characteristic frequency component is defined as the damped component with the lowest resonance frequency. The characteristic frequency component is only taken into consideration because the other resonance frequency components not only have much smaller amplitude than the characteristic frequency component, but also can be almost completely eliminated by a low-pass filter.
Generally, the longer a fault distance is, the lower a characteristic frequency becomes. When a fault occurs at a short distance from a relaying point, the characteristic frequency is so high that a low-pass filter used for anti-aliasing can almost completely attenuate the characteristic frequency component. On the other hand, when a fault occurs at a long distance from a relaying point on a long transmission line, the characteristic frequency is not so high compared to the cutoff frequency of the low-pass filter. Accordingly, the characteristic frequency component is insufficiently attenuated by the low-pass filter and has an adverse influence on the measurement of the fundamental frequency component. Therefore, in addition to the DC-offset, the characteristic frequency component should be taken into consideration when measuring the fundamental frequency component by using orthogonal transforms or other techniques.
It is an object of the present invention to provide a method for measuring the fundamental frequency component of a fault current or voltage signal distorted with the DC-offset, the characteristic frequency component and harmonics in a protective relaying system.
To achieve the above objective, a method for measuring the fundamental frequency component of a fault current or voltage, comprises the steps of:
measuring a fault current or voltage signal, which includes a DC-offset, a characteristic frequency component, a fundamental frequency component, and harmonics, with a predetermined sampling period(S10);
filtering the fault current or voltage signal to remove the harmonics(S20);
removing the fundamental frequency component from the filtered fault current or voltage signal(S30);
calculating the DC-offset and the characteristic frequency component with the fault current or voltage signal wherein the fundamental frequency component is removed(S40); and
determining the fundamental frequency component by subtracting the calculated DC-offset and characteristic frequency component from the filtered fault current or voltage signal.
In the step of filtering the fault current or voltage signal to remove the harmonics, the sine filter is preferably used.
In the step of removing the fundamental frequency component, the linear filter is preferably used.
In the step of calculating the DC-offset and the characteristic frequency component, Prony""s method is preferably used.